On account of the high thermal and mechanical stresses in gas turbines, it is necessary to supply most of the rotor blades and stator blades of the turbines with cooling air. In order to ensure correct cooling of the blades, a sufficiently high pressure ratio between cooling air and hot gas in the hot gas passage is necessary. As cooling air, mainly air is bled at bleed points of the gas-turbine axial compressor which are provided for it. By means of various piping systems, this is then fed to the individual turbine blades. An important point in the gas turbine design in this case is the correct selection of the pressure level of the bleed points to match the required pressure level of the turbine at the points which are to be cooled.
A greatly simplified schematic diagram for a gas turbine which is cooled in this way is reproduced in FIG. 1. The gas turbine 10 of FIG. 1 comprises a compressor 12, which, via an air intake 11, inducts and compresses air, a combustion chamber 13, in which a fuel 14, using the compressed air, is combusted and a hot gas is produced, and a turbine 15 equipped with turbine blades, in which the hot gas is expanded, performing work, and then discharged via an exhaust gas outlet 17, wherein for cooling the turbine 15 compressed cooling air is tapped from the compressor 12 and fed to the turbine 15 via a cooling air line 18. The bleed and feed can be carried out in this case at different points of the compressor 12 or of the turbine 15. For reducing the cooling air temperature, a cooler 19 can be additionally arranged in the cooling air line 18.
In the rear turbine stages in the flow direction, a rather low pressure level prevails. Furthermore, the thermal loads are mostly so low that active cooling of these stages is seldom necessary. Yet if active cooling of the last stages is required, for cost reasons a bleed point on the actively cooled blades which is operating at the next higher pressure level is used in most cases. Since this pressure level, however, lies above that which is actually required and the pressure is throttled in order to minimize the volume of cooling air, in this respect more work than is necessary is performed during the compression of the fluid. This additional work, as a result of the throttling of the pressure, occurs as a loss in the balance.
The provision of a separate bleed point or the utilization of an external cooling air source (compressors, fans, etc.) does not make any sense in most cases for cost reasons.
Printed publication U.S. Pat. No. 2,578,481 discloses a gas turbine in which an intermediate casing is located between the compressor and the turbine and in which a radially acting auxiliary compressor is arranged on the rotor and cooling air, under pressure, is delivered into the intermediate casing which serves for cooling a bearing of the rotor shaft, the rotor disk of the turbine and also the outer carrier ring for the rotor blades. Cooling of the stator blades and/or rotor blades of the turbine by means of the auxiliary compressor is not provided.
From printed publication U.S. Pat. No. 4,005,572 it is known to arrange auxiliary blades on the rear side of a turbine rotor in a gas turbine in order to induct additional cooling air and to introduce it into the turbine together with the hot gases. Also, cooling of the blades of the turbine by means of the auxiliary blades is not provided in this case.